The present invention relates to a fuel cell comprising a polymer electrolyte used for portable power sources, electric vehicle power sources, domestic cogeneration systems, etc.
A fuel cell comprising a polymer electrolyte generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. This fuel cell is basically composed of a polymer electrolyte membrane for selectively transporting hydrogen ions and a pair of electrodes disposed on both surfaces of the polymer electrolyte membrane. The electrodes comprise a catalyst layer and a gas diffusion layer which is formed on the outer surface of the catalyst layer and which has both gas permeability and electronic conductivity. The catalyst layer is formed of a mixture of a carbon powder carrying a platinum group metal catalyst and a hydrogen-ion conductive polymer electrolyte, and the gas diffusion layer is composed of, for example, a carbon paper subjected to a water repellency treatment.
In order to prevent outward leakage and intermixing of the supplied fuel and oxidant gases, gas sealing materials or gaskets are arranged so as to encompass the electrodes and sandwich the polymer electrolyte membrane. These sealing materials or gaskets are combined integrally with the electrodes and polymer electrolyte membrane. This is called “MEA” (electrolyte membrane electrode assembly). Disposed outside the MEA are electrically conductive separator plates for mechanically securing the MEA, and at the same time, interconnecting adjacent MEAs electrically in series. The separator plates have, at a portion to come in contact with the MEA, a gas flow path formed for supplying a reactant gas to the electrode and removing a generated gas and a surplus gas. Although the gas flow path may be provided separately from the separator plates, grooves are usually formed on the surfaces of the separator plates to serve as the gas flow path. In a general structure of the fuel cell, the MEAs, separator plates and cooling sections are alternately stacked to form a stack of 10 to 200 cells, the cell stack is sandwiched by end plates with a current collector plate and an insulating plate interposed between the cell stack and each end plate, and the resultant is clamped with clamping bolts from both sides.
In such a polymer electrolyte fuel cell, the separator plates are often composed of a flat carbon plate which has, on a portion to come in contact with the anode or cathode, a gas flow path for supplying the fuel gas or oxidant gas to the anode or cathode. The separator plates are commonly flat, without having any difference in height between the portion on which the gas flow path is formed and its peripheral portion which is to come in contact with a surface of the gaskets sandwiching the polymer electrolyte membrane.
The use of such separator plates will cause the following problems.
In such a fuel cell, the MEA must be sandwiched by an anode-side separator plate and a cathode-side separator plate such that the gas diffusion layers of the anode and the cathode are in contact with the separator plates while the polymer electrolyte membrane, the anode and the cathode are under appropriate pressure. Also, a pair of gaskets sandwiching the periphery of the electrolyte membrane must be compressed by the anode-side and cathode-side separator plates so as to seal the periphery of the MEA. However, when the separator plates are flat as described above, i.e., when the portion of the separator plate in contact with the anode or the cathode and the portion of the separator plate in contact with the gasket are on the same plane, the degree of compression of the gaskets determines the degree of contact between the separator plates and the gas diffusion layers (the term “the degree of compression of the gaskets” as used herein refers to the gasket thickness to be reduced by compression, or the difference in gasket thickness between before and after the gaskets are compressed). Thus, in order to ensure sufficient contact between the separator plates and the gas diffusion layers and therefore minimize the electrical resistance therebetween, it is necessary to make the degree of compression of the gaskets extremely large when the gas diffusion layers are formed of a soft material such as carbon paper.
Further, in order to reduce the thickness of the MEA and therefore reduce the size of the fuel cell stack, the gas diffusion layers to be used in the MEA need to be thinner than the conventional ones. However, since the conventional separator plates are unable to sufficiently compress the gaskets while ensuring sufficient contact with the gas diffusion layers, it has been difficult to make the MEA thinner than the conventional one. Thus, the conventional separator plates have another problem in that they are unable to be applied to a thinner MEA.